Rapid acquisition of ultra-wideband radio signals and implementation issues of closed -loop multiple-antenna systems
Ultra-wideband radio (UWB) and multiple-input multiple-output (MIMO) communication systems are two of the key architectures that make high-speed wireless communication systems a reality. UWB has been adopted as the physical layer solution for IEEE wireless personal area networks (WPAN), and due to desirable spectral properties, it may play a prominent role in future battlefield networks. The MIMO technique is widely utilized in numerous applications such as high throughput wireless local area networks (WLAN), mobile wireless metropolitan area networks (WMAN) and the third generation (3G) and fourth generation (4G) cellular communication systems. In the dissertation, we discuss several critical aspects of the system design of each of these architectures.
Impulsive UWB radio provides many promising features for wireless communications in a dense multipath environment. However, these features are largely the result of the enormous effective processing gain, which can make acquisition difficult at the receiver. In this dissertation, a recently developed theory of minimum complexity sequential detection is applied to the hybrid acquisition problem. As in previous hybrid schemes, a number of potential timing phases are checked as a group; however, a phase is disregarded as soon as it appears unlikely rather than waiting for a "winner'' to be chosen from the group. Another phase then replaces the disregarded one. Analysis and simulation results indicate that the proposed scheme can improve average acquisition times for highly spread systems operating over either additive white Gaussian noise (AWGN) or multipath fading channels.
Channel state information at the transmitter, even imperfect, can improve the channel capacity of a wireless link employing multiple antenna elements at both the transmitting and receiving ends. More meaningfully, joint design of the transmitter and receiver is possible to reduce the receiver complexity. Eigenbeamforming has been proven to be the precoding scheme that not only achieves the channel capacity, but also optimizes the error performance or minimizes the transmit power. This dissertation presents a novel adaptive modulation scheme that loads power and integer bits over the eigenmodes according to the outdated CSI at the transmitter. We consider an uncoded narrowband MIMO system in a time-varying and spatially uncorrelated fading channel. It is demonstrated numerically that the scheme disregarding the time-varying nature of the channel misses the bit error rate (BER) target significantly, while the proposed scheme meets the BER requirement for every interested correlation between the outdated and current CSI. Moreover, simulations also indicates that the propose scheme provides much higher spectral efficiency than the strategy that adds energy margin to achieve the target error performance.
Orthogonal frequency division multiplexing (OFDM) converts a wideband channel into a number of parallel flat fading subchannels; thus, it avoids complicated time-domain equalization. The combination of MIMO technology and OFDM (MIMO-OFDM) enjoys significant data rate gains over single-antenna, single-carrier systems in multipath fading channels. Hence, it has gained significant popularity in high speed wireless communication systems. However, multiplexing a large number of independent subchannels results in a very high peak-to-average power ratio (PAPR). Transmitted signals with high PAPR pose a serious problem for practical communication systems. Unless this issue is successfully addressed, OFDM advantages in low-cost communication systems are seriously compromised. In multiple-antenna systems, the high signal peak may occur in any of the transmit antennas. Moreover, in closed-loop systems, the adaptive spatially precoded signals on each antenna are no longer standard phase shift keying (PSK) or quadrature amplitude modulation (QAM) symbols, which makes peak control more difficult. Some techniques developed for single-antenna OFDM can be readily applied to MIMO-OFDM. However, degrees of freedom in space are not exploited. In this dissertation, a novel PAPR reduction scheme is presented for a closed-loop MIMO-OFDM system. Spatial subchannel that are rejected for data transmission are utilized for the purpose of peak reduction.